Continued effort has been made to investigate the model systems of the heme iron (III) in cytochrome P-450. On the experimental side, comparative EPR (electron paramagnetic resonance) studies were carried out for two different model systems and the various mammalian liver microsomal P-450. On the theoretical side, we have shown that the lowest excited 4T2 state, which has not usually been included in the crystal field analysis of 2d-electron states of the high spin Fe(III) complexes, plays an important role in the systems with a substantial rhombic distortion. Other (collaborative) works are described. BIBLIOGRAPHIC REFERENCES: Rispin, A. S., Kon, H. and Nebert, D. W.: Electron spin resonance study of oxygen-17 enriched oxybenzo(a)pyrene radical. Mol. Pharm. 12: 476-482, 1976. In press. Sato, M., Kon, Akoh, H., Tasaki, A., Kabuto, C. and Silverton, J. V.: Anomalous magnetic properties of tetraphenylporphinato Co(II) complex in the solid state. Chem. Phys. 16: 405-410, 1976. In press.